gui2/twml_error: Ignore empty _info.cfg [info] title= attribute

This allows the synthetic add-on name to be used in the absence of an
[info] title= attribute, e.g. for add-ons downloaded with older versions
of Wesnoth or wesnoth_addon_manager.

changelog

@@ -65,6 +65,9 @@ Version 1.13.0-dev:
    * Added a button to copy the currently displayed content from the :inspect
      dialog to clipboard.
    * Added a button to copy the in-game Chat Log dialog contents to clipboard.
+   * Fixed the WML load error dialog not displaying an add-on name instead of
+     falling back to its directory name if the add-on contains an outdated
+     _info.cfg file lacking an [info]title= value.
  * WML engine:
    * Added customizable recall costs for unit types and individual units,
      using the new recall_cost attribute in [unit_type] and [unit].

src/gui/dialogs/wml_error.cpp

@@ -110,7 +110,7 @@ std::string format_file_list(const std::vector<std::string>& files_original)
 
 			const config& info_cfg = cfg.child("info");
 
-			if(info_cfg) {
+			if(info_cfg && !info_cfg["title"].empty()) {
 				file = info_cfg["title"].str();
 				continue;
 			}